Exposure control in xerographic printing



May 23, 1967 F. URBACH 3,321,307

EXPOSURE CONTROL IN XEROGRAPHIC PRINTING Filed July 15, 1963 FIELD METERAMPLIFIER 1 FIG FRANZ URBACH 70 n7, V 8 f2 INVENTOR.

7/ ELECTRO I METER BY ATTOR/VE Y5 United States Patent 3,321,307EXPOSURE CONTROL IN XEROGRAPHIC PRINTING Franz Urbach, Rochester, N.Y.,assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of NewJersey Filed July 15, 1963, Ser. No. 295,143 Claims. (Cl. 96-4) Thepresent invention relates to xerography and particularly to a method andapparatus for controlling the exposure of a charged photoconductorsheet.

It is the object of the invention to provide apparatus for automaticallyterminating the exposure of a xerographic plate when the electrostaticimage being formed by the exposure has reached approximate optimumquality. Optimum quality of an electrostatic image is that distributionof charges which, when developed with a suitable toner, produces printswhose quality is the optimum obtainable with the particularphotoconductor and toner being used. As is well known, the quality in aprint refers to the minimum and maximum densities and to the contrast ofthe print. High quality prints have extremely low minimum density (cleanhighlights), fairly high maximum density and a contrast whichapproximates or slightly exceeds that of the original subject beingreproduced.

The present invention is also particularly useful in the copying ofdocuments.

A further object of the invention is to provide method and apparatus forcontrolling the termination of the exposure time. It is a particularobject of the invention to provide such control independent of theintensity of the exposure over a wide range of intensities, includingall or practically all intensities found in light transmitted throughcommonly over-exposed and under-exposed transparencies (negatives orpositives). It is also a particular object of the invention to provide asystem which corrects for variations in sensitivity of the recordingmaterial at the same time as it corrects for the density of thetransparency being printed.

The present invention is applicable to the various forms of xerographyincluding those in which the toner adheres to the charged areas andthose in which the toner adheres to the discharged areas of theelectrostatic image. It is equally applicable to systems in which thetoner image remains on the photoconductor (zinc oxide in resin on paperbase is sufiiciently inexpensive) and those in which the image istransferred from a reusable photoconductor (which may be relativelyexpensive) to a separate receiving sheet. The invention utilizes thechange in an average electric field adjacent to the photoconductor asthe charge on the photoconductor changes from a uniform, relativelyhigh-density charge to an imagewise distribution of charge with a loweraverage density. The proper average electric field is attained by anexposure time which depends on the intensity of the exposing image andthe sensitivity of the photoconductor.

While it is true that different subjects will have slightly differentaverage fields associated with the optimumquality electrostatic imagesthereof, it turns out in practice that a statistically very high yieldof optimum-quality prints are obtained when variations in subject matterare neglected or ignored. The present invention for any one subjectgives optimum-quality prints from negatives which range from two stopsunder-exposed to four stops over-exposed. Furthermore, setting thesystem to give such optimum results for an average subject of the typeto be reproduced gives acceptable quality for practically all subjectsof that same general type. The setting is, of course, somewhat differentwhen used for document copying, for example from microfilm, than whenused for continuous-tone printing; and the setting is diiferent fordifferent types of toners, but over a wide range of types andsensitivities of photoconductors the setting is unaflected.

According to the present invention, the electric field, or more exactlythe change in the electric field, adjacent to a substantially uniformlycharged photoconductor is measured during the exposure to an image. Theexposure is started either manually or by the measuring system. Theexposure efiectively removes charges in the exposed areas, causing thefield to change, and the exposing is terminated when the measured fieldor the change in field reaches a predetermined value. As pointed outabove, excellent quality prints are obtained from a wide range ofnegatives and a wide range of photoconductor sensitivities when such acontrol system is used.

The term effectively removes charges is used herein to include movementsof charges from the surface, move ments of opposing charges to or nearerthe surface and what is sometimes referred to as a disorientation ofdipoles. The present invention is not concerned with the theory orconvention adopted, but uses the term in its commonly accepted sense.

The charging of the surface must be terminated before or at the momentexposure starts. In practice, the surface is charged uniformly andbrought to the position for exposure before the measuring electrometeror field meter is rendered operative. For example, the electrometer mayhave its field-sensing electrode shorted to ground or shielded from thecharged photoconductor until the photoconductor is in place and readyfor exposure. When the shield is used and then removed, the sensingelectrode of the electrometer or field meter is immediately subjected toa high electric field. When the grounded electrode is used and thenungrounded, it remains at ground potential until the field of thecharged photoconductor surface changes either through spontaneous decayor exposure. The exposing light is then turned on or the shutter in theprojection printer is opened either manually or by a relay operated bythe output of the electrometer or field meter. Then, as the field falls(causing, in the case of the removed shield, a decrease of potential onthe sensing electrode and in the case of an unground electrode anincrease of the potential thereof) the measuring is used to terminatethe exposure at a predetermined value of the average electric field orat a predetermined change in the field, namely that correspondingapproximately to an optimum quality electrostatic image. It should benoted, at this point, that one of the advantages of the presentinvention arises from the fact that it takes into account any reasonableamount of natural leaking away of the electrostatic charges as well asthe discharging due to the image exposure. While excessive naturalleakage of charge will tend to degrade the image, the present inventionstill assures the optimum available among such degraded images.

When the present invention is used for document copying, the type ofdevelopment may determine whether the meter should be set to terminatethe exposure at a predetermined value or at a predetermined change fromthe initial value. The latter is particularly useful for any type ofdevelopment which takes place with no grounded electrode near therecording surface at the time of development; this includes fringedevelopment by powder cloud, liquid development or cascade developmentwith insulating carrier for the toner. The exposure is terminatedsubstantially at a fixed field value when the development is to be inthe presence of a grounded electrode as for example with magnetic brushdevelopment or some forms of liquid or powder cloud development with agrounded electrode adjacent the image surface.

Meters of two general types are well-known and com mercially availablefrom various manufacturers. Each of the two types has its advantageswhen used with the present invention.

One type is superior to the other when the sensing electrode usedtherewith is a transparent one held immediately in front of the chargedsurface of the photoconductor being measured. One form of electrometeruseful with this type of sensing electrode is the General Radio D.C.Amplifier and Electrometer, Type 1230A. The sensing electrode beingtransparent does not obscure the printing beam.

A second type of meter commonly used employs a mechanical chopperbetween the source of the field and the sensing electrode. The chopperand sensor constitute two sectored discs or vanes, one of which isstationary and the other rotating behind or preferably in front of thestationary one. A steady field creates an alternating signal in thesensing electrode. The front vane (either the chopper or the fixed one)is grounded, or biased to a fixed potential, and the other one acts asthe sensing electrode supplying an AC. signal to the field meteramplifier. Since such a sensing electrode with chopper is normallyopaque, it is positioned, in the present invention, to one side of theimage-forming beam so as not to obscure the beam. The sensing electrodein this case faces the surface to be measured, obliquely.

I have found that such oblique measurements of the average field aresatisfactory for the purposes of the present invention.

Electrometers or field meters of the rotating electrode type aredescribed in many publications. An elementary description appears, forexample, in Measurements of Electrical Polarization in Thin DielectricMaterials by Tyler, Webb and York, Journal of Applied Physics, vol. 26,pp. 61-68, January 1955. Also, the five references listed in a footnoteon page 56 of this Tyler et al. article describe useful forms of suchelectrometers. The various known forms of electrometers have, of course,different ranges of current values in their output or measuringcircuits. If these values are not sufficient to operate the relays orthe equivalent involved in initiating and terminating the exposure,various degrees or stages of amplification are introduced.

Other objects and advantages of the invention will be fully understoodfrom the following description when read in connection with theaccompanying drawing in which:

FIG. 1 schematically illustrates a preferred embodiment of theinvention.

FIG. 2 similarly illustrates the field meter or electrometer employed inFIG. 1.

FIG. 3 illustrates a modification of one part of the arrangement shownin FIG. 1 to incorporate a different embodiment of the invention.

In FIG. 1, light from a lamp illuminates a transparency 11 and an imagethereof is focused by a lens 12 on the uniformly charged surface of aphotoconductor 15 carried on a conducting support 16. A shutter for theoptical system is indicated schematically at 13. The lamp is energizeddirectly from a 110-volt A.C. source 17 when switch 18 and relay contact19 are closed. The relay contact 19 is normally open and is closed by arelay solenoid 20 when the latter is energized. If the switch 18 isalready closed, closing of the relay contact 19 initiates the exposure;alternatively the exposure can be started manually by closing the switch18 after the relay contact 19 is closed by the solenoid.

According to the invention, a sensing electrode is positioned to oneside of the printing beam and is arranged to face the charged surface ofthe photoconductor 15 obliquely. The sensing electrode 25 is stationarybehind a rotating sector blade or chopper 26 of an electrometer of theabove-discussed type. The chopper 26 is biased slightly above or belowground by a battery,

shown schematically at 27, in order to permit a zero adjustment to bemade on the recording means. The rotating chopper alternately shieldsand exposes the stationary elect-rode 25. This mechanical chopping ofthe field produces an AC. potential across a high resistance 28.

In the operation of the device, a metal shield 31 is placed across theilluminating beam and in front of the sensing electrode 25 of the fieldmeter, until a fully charged photoconductor 15 is in place, as shown,and ready for exposure. This shield 31 maintains the field meterinoperative, i.e. prevents the field meter from measuring the averagefield adjacent to the photoconductor 15 until the photoconductor sheetis ready to receive the exposure. The shield 31 is then removed manuallyby grasping the extension 32 and withdrawing the shield from the metalhousing 33 which is grounded and which shields the whole unit. Insteadof the simple extension 32, mechanical means can be provided forremoving the shield or for moving it to an inoperative position,rendering it ineffective. When the shield is removed or renderedineffective, the electrometer sensing electrode becomes operative, and asignal acnoss the resistor 28 is amplified in the field meter amplifier36 and impressed across a resistance 37 which in this case is a 500-0hmresistance.

Standard field meter amplifiers such as 36 are provided with outputconnectors, one of which is grounded and across which the variableresistance 37 is connected. With a field meter of the type describedbelow having a final rectifier (such as 57, FIG. 2) which may beconnected with either polarity, the output can be a negative,negative-going signal for an increasing positive surface potential, orwith a reverse arrangement, the output can be a negative, negative-goingsignal for an increasing negative surface potential. With a SOD-ohmresistor at 37 the output of the field meter 36 produces for example apotential acnoss this resistor 37 between zero and approximately 0.5volt as the magnitude of the surface potential under measurement isincreased.

The following example is given merely for clarity; it describes theoperation of the amplifier for one group of settings of the controls.With no input signal, the 6 BH6 tube 40 has its control grid at +0.04volt from ground. The cathode is also above ground by the potential dropacross the cathode resistor. In this condition the tube 40 is conductingfairly heavily. The potential at the plate is +62 volts which is droppedto +35 volts by the Zener diode (IN205). This potential drop plus thepotential drop across a portion of the resistor 44 puts the grid of the6C4 tube 43 at 16 volts which is sufficient to keep the tube 43 cut offand no current flowing through the relay 20. As mentioned above, anegative, negative-going output current from the field meter amplifieris produced as a sensed field strength from the photoconductor 15increases. As increasing negative current passes through the resistor37, the potential on the grid of tube 40 becomes more negative. I-tsplate current then decreases, causing the plate potential to rise. Whenthe grid of tube 40 reaches -0.5 volt from ground, the plate of thistube 40 is at +84- volts. The diode 42 drops this to +56 volts and tube43 grid is then at 2.0 volts, as determined by the ad justment of thecontact on resistor 44. This grid voltage permits sufficient current toflow in the plate circuit to actuate the relay 20, closing the relaycontact 19 and starting the exposure if switch 18 is already closed.This energized condition of relay 20 is reached immediately after theshield 32 is removed, thus exposing the sensing electrode 25, 26 to thefield adjacent to the charged photoconductor 15.

Immediately upon starting the exposure, the surface potential on thephotoconductor 15 starts to decrease, which causes the potential on thegrid of the tube 40 to rise, increasing the plate current and causingthe plate potential to drop, causing the grid on tube 43 to be come morenegative reducing the plate cur-rent through relay 20 until it releases.This occurs when the grid potential on tube 40 has risen to -0.2 voltfrom ground, at which time the plate potential is 70 volts. This isdropped to +42 volts by the Zener diode 42, and the grid of the tube 43is at -11 volts at the point selected on the resistor 44. This is justsuflicient to permit the relay 20 to release, terminating the exposure.

Instead of using the shield 31, the sensing electrode 25 may be groundeduntil the sheet 15 is ready for exposure. Opening the ground connectionrenders the sensing electrode 25, 26 operative, starting the exposurewhich, as described above, terminates when the potential on the grid ofthe tube 43 reaches -2.0 volts. It is customary to use a vacuum plate ofthe type commonly used in process photography, to hold the plate 15 flaton the shielded easel of the projection printer illustratedschematically in FIG. 1. After the exposure is terminated, thephotoconductor sheet 15 is developed or toned by any standardxerographic method; the toner image may be fused to the photoconductoror transferred to a receiving sheet.

As shown by broken lines in FIG. 1, the lamp may remain on, and theexposure may be controlled by opening and closing the shutter 13 bymeans of a shutter control actuated by the closing and opening of theswitch 19.

It should be noted that this instrument according to the presentinvention not only corrects for various densities of the transparency 11or intensities of the lamp 10, but also corrects for sensitivities inthe layer 15. In one experiment, photoconduc-tors whose sensitivitiesvary by as much as 7 to l were used. Each coating was charged to asurface potential of -400 volts under a --9 kv. corona, before exposure.The optimum exposure time was determined for one of the coatings. Aseries of prints were run on the various coatings and another serieswere run with a 1.0 neutral density filter inserted in the printer beam.Without interference on the part of the operator, the apparatus wasallowed to monitor each of the exposures. When the same average surfacepotential had been reached, the exposure was automatically terminated.The resulting prints with all of these variations were essentiallyidentical and for practical purposes were indistinguishable. They wereall developed with a standard magnetic brush toner, by magnetic :brushdevelopment.

As a second experiment, a color transparency was exposed through primaryred, green and blue filters to make separations. The automatic controlillustrated in FIGS. 1 and 2 properly corrected the exposure times andin the particular test being described, the exposure times turned out tobe red, 5.3 seconds; green, 11.4 sec oncls; and blue, 29.5 seconds.Except for the differences due to the colors, the separation prints,after toning with the same developer, appeared to have about the samecontrast and average density.

FIGS. 1 and 2 are merely for illustration of a suitable circuit, sinceany field meter may be used, and any of the many known circuits foroperating a relay from the output of a field meter may be used withoutdeparting from the spirit of this invention.

In FIG. 2 essential features of one particular field meter 36 areillustrated. The input potential from a point or tap on the l-megohmresistor 28 is fed to two stages of amplification in tubes 50 and 51(l2AU7) and the output of the last stage is impressed across a110,000-ohm resistor 52. A voltage signal tapped off this resistor 52 isthen fed to the grid of tube 54 (6C4) which is tuned by the tunedcircuit 53, the output of which is fed to the control grid of 6AK6pentode 55 Whose output current is sufficient through a transformer 56and a rectifier 57 to give a substantial signal across a SOD-ohmresistor 37. From this point on, as shown in FIG. 1, there is furtheramplification to obtain sufficient current to operate the relay 20. Thevalue of the impedance of the load resistor 37 is selected to match theparticular electrometer or field meter being used.

In FIG. 3, the easel of the projection printer holding the chargedphotoconductive surface 15 is shielded by a shield 70. In this case, thesensing electrode 71 of the electrometer 72 is transparent. A differenttype of field meter is used, and hence the output is across a resistor74 which is equivalent to resistor 37 of FIG. 1 but is selected to haveproper impedance value. The potential at a selected point on theresistor 74 is fed to an amplifier tube such as 40 in FIG. 1. In FIG. 3,the sensing electrode 71 is placed within a fraction of a millimeter ofthe photooonductor surface and uniformly covers the whole surface. Thefield meter is maintained inoperative by closing the grounding switch 73until the charged plate 15 is in position ready for exposure. Due to thecharge on the photoconductor surface, charges of opposite polarity areinduced on the sensing electrode while it is maintained at groundpotential. When the switch 73 is opened the sensing electrode isungrounded; the electrometer still indicates ground potential. Theexposure is then started causing the charge on the photoconduc-tor to beeffectively removed leaving an excess of charges on the sensingelectrode so that its potential rises and this rise is indicated on theelectrometer. When the potential changes a predetermined amount orreaches a predetermined value, the exposure is terminated.

It should be noted that the present invention requires the average ofthe field over a substantial area of image to be measured. Accordingly,the preferred embodiments of the invention expose the whole of theexposure areas of the photoconductor, at one time. Scanning exposure ofthis area complicates the operation of the present invention and istherefore to be avoided.

It should be further noted that the closing and opening of the relaycontact 19 can be manual or relay contact 19 can be omitted. That is,the exposure is initiated manually and when the operator (reading thefield value on the miliiammeter which is a part of commerciallyavailable field meters, in series with the load impedance 37) notes thatthe field has dropped to the optimum image value, he opens the switch 19or 18, terminating the exposure. The preferred embodiments are theautomatic ones illustrated, however.

Having thus described the preferred embodiments of my invention, it willbe understood that variations and modifications can be effected withinthe spirit and scope of the invention as described hereinabove and asdefined in the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a xerographic process for controlling the exposure of asubstantially uniformly charged photoconductor, the steps whichcomprise: exposing said photoconductor image'wise while simultaneouslymeasuring the changes in the electric field as charges are effectivelyremoved in exposed areas, and terminating said exposing when themeasured change in the electric field reaches a predetermined value.

2. In a xerographic process, the steps of uniformly charging to a fixedfield value, in the absence of actinic radiation, the surface of aphotoconductor, exposing said surface imagewise while simultaneouslymeasuring the change in the electrical field as charges are effectivelyremoved in exposed areas, and terminating said exposing when themeasured field falls to a predetermined value.

3. In a xerographic process, the steps of uniformly charging, in theabsence of actinic radiation, the surface of a photoconductor, placing agrounded electrode adjacent said photoconductor, ungrounding saidelectrode, exposing said surface imagewise while simultaneously meas- 7uring the potential of said electrode as charges are effectively removedin exposed areas, and terminating said exposing when said potential haschanged by a predetermined amount.

4. A projection printer for imagewise exposing a substantially uniformlycharged surface of a photoconductor sheet comprising means forprojecting a light beam onto said surface with an image in focus,

a field meter with its sensing electrode facing said surface but notobscuring said beam for measuring the average electric field adjacent tothe surface, and means controlled by said field meter for terminatingsaid projecting means when the field decreases to a value equal to thatof an electrostatic image of approximately optimum quality.

5. A printer according to claim 4 including means for maintaining saidfield meter inoperative to measure said average field until saidphotoconductor sheet is ready to receive said exposure, and means forrendering said maintaining means ineffective and hence said field meteroperative.

6. A printer according to claim 5 in which said maintaining means is anelectric ground connected to said sensing electrode and disconnectabletherefrom by said rendering means.

7. A printer according to claim 5 in which said maintaining means is ashield removably located between the sensing electrode and said surfaceand removable by said rendering means.

8. A printer according to claim 4 in which said sensing electrode istransparent and uniformly spaced from said surface.

9. A printer according to claim 4 in Which said sensing electrode is toone side of said beam and faces said surface obliquely.

10. A projection printer for imagewise exposing a substantiallyuniformly charged surface of a photoconductor sheet comprising means forprojecting a light beam onto said surface with an image in focus,

an electrode adjacent to said surface,

means for grounding and ungrounding said electrode,

means for measuring the electric potential of said electrode, and

means controlled by said measuring means for terminating said projectingmeans when said potential has changed from ground a predeterminedamount.

References Cited by the Examiner UNITED STATES PATENTS 2,297,691 10/1942Carlson 96--1 2,781,705 2/1957 Crumrine et al -1.7 3,013,203 12/1961Allen et al 324-32 3,251,685 5/1966 Bick'more 96--1 NORMAN G. TORCHIN,Primary Examiner.

J. TRAVIS BROWN, Examiner.

C. E. VAN HORN, Assistant Examiner.

1. IN A XEROGRAPHIC PROCESS FOR CONTROLLING THE EXPOSURE OF ASUBSTANTIALLY UNIFORMLY CHARGED PHOTOCONDUCTOR, THE STEPS WHICHCOMPRISE; EXPOSING SAID PHOTOCONDUCTOR IMAGEWISE WHILE SIMULTANEOUSLYMEASURING THE CHANGES IN THE ELECTRIC FIELD AS CHARGES ARE EFFECTIVELYREMOVED IN EXPOSED AREAS, AND TERMINATING SAID EXPOSING WHEN THEMEASURED CHANGE IN THE ELECTRIC FIELD REACHES A PREDETERMINED VALUE. 4.A PROJECTION PRINTER FOR IMAGEWISE EXPOSING A SUBSTANTIALLY UNIFORMLYCHARGED SURFACE OF A PHOTOCONDUCTOR SHEET COMPRISING MEANS FORPROJECTING A LIGHT BEAM ONTO SAID SURFACE WITH AN IMAGE IN FOCUS, AFIELD METER WITH ITS SENSING ELECTRODE FACING SAID SURFACE BUT NOTOBSCURING SAID BEAM FOR MEASURING THE AVERAGE ELECTRIC FIELD ADJACENT TOTHE SURFACE, AND MEANS CONTROLLED BY SAID FIELD METER FOR TERMINATINGSAID PROJECTING MEANS WHEN THE FIELD DECREASES TO A VALUE EQUAL TO THATOF AN ELECTRSTATIC IMAGE OF APPROXIMATELY OPTIMUM QUALITY.